1. Field of the Invention
The present invention relates to a dual band voltage-controlled oscillator circuit which stably oscillates in different frequency bands.
2. Description of the Related Art
An oscillator circuit according to the related art, which is used in an FM or TV audio receiver or a local oscillator of a dual band-compliant cellular phone, will be described with reference to FIG. 9. A trimmer capacitor 1 is interrupted by a switch 2 and is connected parallel to a series circuit of an oscillating coil 3 and a coil 4 when the switch 2 is grounded. The switch 2 is also used to short-circuit the coil 4. The series circuit of the oscillating coil 3 and the coil 4 is connected parallel to a series circuit of a padding capacitor 8 and a variable capacitor 5. The padding capacitor 8 is short-circuited by a switch 6.
A base of an oscillating transistor 10 is connected to one end of the padding capacitor 8 through a capacitor 9. A capacitor 14 is connected between the base and an emitter of the oscillating transistor 10, and a capacitor 12 is connected parallel to the capacitor 14 to be interrupted by a switch 13. The emitter of the oscillating transistor 10 is grounded through a resistor 18 and a capacitor 17. A resistor 11 is provided between a collector and a base of the oscillating transistor 10. The collector thereof, to which power is applied through a resistor 16, is grounded through a capacitor 15. The base thereof is grounded through a resistor 19. An oscillation output is supplied to a mixer through a capacitor 7.
In this local oscillator, the capacitor 12 is connected parallel to the capacitor 14 between the base and the emitter of the oscillating transistor 10 by the switch 13 at the time of receiving an FM band, thereby ensuring an oscillation frequency and realizing high stability. At the time of receiving one of TV channels of 4 to 12, the switch 2 is connected to a TV terminal to be grounded. The switch 6 is turned off, and thus a lower limit frequency is limited by the padding capacitor 8. At the time of an FM band, the switch 2 is connected to an FM terminal, and thus the trimmer capacitor 1 is grounded. Further, the switch 6 and the switch 13 are turned on. If the switch 6 is turned on, the capacitor 12 is connected between the base and the emitter of the transistor 10, whereby stable oscillation is realized.
In the above-described oscillator circuit, in a band having a low oscillation frequency, a value of negative resistance on the oscillating transistor is insufficient, and thus oscillation stability is lowered. Accordingly, in the band having the low oscillation frequency, in order to ensure oscillation stability, a capacitance value between the base and the emitter of the oscillating transistor is made large. To this end, however, a switch and a circuit for controlling the switch are additionally required, which complicates the oscillator circuit.